Submitted to: Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 20, 2006
Publication Date: February 20, 2006
Citation: Wilson, C., Liu, X., Lesch, S., Suarez, D.L. 2006. Growth response of major usa cowpea cultivars. II. Effect of salinity on leaf gas exchange. Plant Science. 170:1095-1101. Interpretive Summary: Over the last several years, there has been increasing interest in amending the soil using cover crops, manures, composted yard wastes, and other organic matter. Covers crops are important in maintaining soil productivity and environmental quality. More recently, this interest has expanded to include desert agriculture. The desert valleys of Southern California have become a proficient producer of a large variety of vegetables. Although hot and arid, these low-desert valleys are well suited to growing high-value fruit and winter vegetables as well as other crops if properly irrigated. Because cowpea is adapted to high temperature and drought, it has become one cover crop of some interest as it may be suitable for growth during summer months in the desert valleys of California when vegetables are not usually grown. In desert agriculture though, salinity is a widespread and prevalent problem. Soil salinity issues are of increasing concern in the Coachella Valley of California where the Colorado River water has been a major source of irrigation water for many years. Additionally, high-quality water for agricultural purposes is becoming increasingly scare due to changing environmental standards and rising demands from urban areas. In an earlier study, we found that certain cowpea cultivars are naturally more salt tolerant than other cultivars. In this study we investigated certain physiological processes which may be involved in this differential response. We measured the effect of salinity on two important physiological processes, photosynthesis and stomatal conductance, of four cowpea cultivars; CB5, CB27, 8517 and 7964, differeing in their ability to grow using saline water for irrigation. We found that salinity had a significant effect on both photosynthesis and stomatal conductance in all four cultivars. However, even though salinity significantly affected these two major physiological process, our analysis indicated this effect is not sufficient to explain the difference in salt tolerance among the cowpea cultivars tested. In this respect, our results are consistent with the notion that salinity tolerance is complex and brought about by a range of physiological procsesses. Future work will investigate the idea that a differential distribution and use of sugars may be involved.
Technical Abstract: A previous study indicated that various cowpea cultivars may be differentially affected by increasing salinity. In an effort to elucidate the physiological processes involved in this differential response, we investigated the effect of salinity on leaf gas exchange of the net photosynthetic rate per unit leaf mass (Pnm), net photosynthetic rate per unit leaf area (Pna), and stomatal conductance (gs) of four major USA cowpea cultivars: CB5, CB27, 8517 and 7964. The experiment was set up as a standard Split Plot design in which the plants were grown in greenhouse sand tanks irrigated with nutrient solution. Seven salinities ranging from 2.6 to 20.5 dS m-1 were constructed from the nutrient solution based on Colorado River water salt composition to have NaCl, CaCl2 and MgSO4 as the salinization salts. The osmotic potential ranged from -0.075 to -0.82 MPa. Light-saturated Pnm, Pna and gs of fully expanded trifoliage were measured using a Li-Cor 6400 Photosynthesis System at two growth stages; 1) during the vegetative growth stage, and 2) during flowering stage. The data were analyzed using SAS ANOVA procedures. We found a highly significant (P less than 0.0001) reduction of Pnm, Pna and gs due to salinity. The responses of Pnm, Pna and gs to salinity could be quantitatively described by the Hoffman/vanGenuchten salt-tolerance model. We found that Pnm is more sensitive to salinity than Pna indicating that salinity may directly affect leaf photosynthetic carbon fixation reactions. Additionally, we found that increasing stomatal closure with increasing salinity may limit Pnm or Pna. While we did not find any significant difference (P greater than 0.05) of Pnm and Pna among the four cultivars, we did find a significant difference (P less than 0.05) in gs. However, no significant salt×cultivar interaction effect (P greater than 0.05) was found on Pnm, Pna and gs indicating that the four cowpea cultivars have the same response pattern of their leaf gas exchange to salinity.